A press furnace of this kind is known from, for example, U.S. Pat. No. 6,303,059. This press furnace is distinguished by a special control method in which a pressing force is detected and the decrease in the speed of penetration into an embedding compound is used as a switch-off signal for the pressing operation.
In the case of a press furnace operating free of faults, this makes it possible to ensure an optimized filling of the hollow cavities and to a certain extent also a secondary compaction of the introduced ceramic dental restoration material. To achieve this, a predetermined switch-off threshold, for example 0.3 mm per 3 minutes, is established, and, if the speed of penetration falls below this value, the press plunger is switched off. To avoid bursting of the embedding compound muffle, the pressing force should lie below a certain value which is at a safety interval from the maximum possible pressing force.
It is true that this method for controlling the speed of penetration is effective per se and has proven itself in practice. However, it would be desirable to further reduce the cycle time for the pressing, without entailing the risk of a poorer restoration result. In this connection, it is particularly important to ensure a sufficient repressing time, since the strength of the dental restoration is mainly defined in this way.
Various measures have been disclosed for optimizing the time at which the furnace is switched off. For example, AU 617064B2 discloses the detection of a pressure increase at a predetermined speed of advance of the press ram. In this solution, however, the plunger speed additionally has to be detected, and, as is known, every additional sensor which is provided, and whose output signal is used for control purposes, detracts from the reliability of the control.
Moreover, it has already been proposed to configure the embedding compound in such a way that it is slightly more elastic and thus less inclined to formation of cracks. This is indeed possible in principle by corresponding provision of a mixture suitable for this purpose. However, a disadvantage is that the hollow spaces that are intended for the formation of the dental restorations are then slightly less accurate in terms of their shape, with the result that the dental restoration parts then often have to be worked in order to remove material, which in turn lengthens the production process.
By contrast, the object of the invention is to make available a method for operating a press furnace and a press furnace, which method and press furnace permit the use of embedding compounds that are especially accurate in terms of their shape, without the pressing force having to be reduced for safety reasons, or without increased risk of bursting of the embedding composition muffle.